1. Field of the Invention
The present invention relates to soft tissue implants, and in particular to a method for promoting tissue adhesion to soft tissue implants.
2. Prior Art
Most foreign bodies entering or contacting living tissue cause the tissue in contact therewith to form an interface with the foreign body comprising inflammatory tissue normally associated with wound healing. Some materials are biocompatible in that they cause minimal and transient formation of inflammatory tissue when implanted into living tissue. Examples of biocompatible materials are: aluminum, carbon, and titanium. Thus, all three are common materials used for hard tissue implants. Since titanium is rather expensive, it is sometimes applied as a coating to another, less expensive rigid material in forming a hard tissue implant.
Polymeric materials can be sufficiently elastic for use as soft tissue implants. However, polymeric materials are generally not sufficiently biocompatible to be used as the portion of an implant that contacts the tissue. Accordingly, carbon or aluminum has been applied to the surface of a polymeric substrate to improve the histocompatibility and vascular graft patency of the resulting implant. The best results have been obtained using a carbon surface coating, which has been applied to polymeric surfaces comprising both hard tissue implants, such as in U.S. Pat. No. 3,952,334 to Bokros et al, and to a soft tissue suture, such as in U.S. Pat. No. 4,149,277 to Bokros. Aluminum also has been vaporized and vapor deposited on a suture made of Dacron in U.S. Pat. No. 3,557,795 to Hirsch. However, being a stiff metal, titanium is not suitable for the design of pliable, soft implant devices which would be required for the development of vascular grafts, percutaneous implants or other materials desired in plastic and reconstructive surgery.
As a coating material, carbon is preferred to aluminum, because aluminium tends to dissolve as it reacts with the surrounding tissue. However, aluminum adheres better to a polymeric substrate, while carbon tends to chip off easily. Since titanium tends to bond chemically with a polymeric substrate, a small proportion o titanium has been mixed with pyrolytic carbon to form a coating on a substrate material such as artificial graphite, boron carbide, silicon carbide, tantalum, molybdenum, tungsten and various ceramics such as mullite. This mixture of carbon and titanium to form a substrate coating for a hard tissue implant is disclosed in U.S. Pat. No. 3,677,795 to Bokros et al.
Titanium is generally recognized as one of the most tissue compatible materials presently available. Titanium has been reported to be a highly histocompatible hard tissue implant material in many applications. Titanium appears to have a stimulatory effect on connective tissue cell proliferation, and cellular adhesion to titanium has been postulated by others. Kasemo, "Biocompatibility of Titanium Implants: Surface Science Aspects," J. Prosth. Dent., 48: 487-494 (1982) and Albrektsson et al, "The Interface Zone of Inorganic Implant in vivo: Titanium Implant in Bone," Ann. Biomed. Engr. 11:1-27 (1983).
The effect on soft tissue ingrowth of altering the surface of an implant has been studied. Perhaps the best known surface alteration is achieved by coating polyethylene terephthalate (PET) with a carbon layer. For example, vacuum evaporating carbon on a velour fabric results in an implant that can be employed as vascular grafts, prosthetic fabrics and prosthetic heart valve sewing rings. Although carbon-coated PET velour fabric vascular grafts showed relatively higher patency rates than the non-coated prostheses, the carbon-coated fibers appeared to show no histological difference in the elicited tissue response when compared to non-coated PET velour fabric fibers. Aluminum also has been evaporated onto the surface of PET velour fabric vascular prostheses, and a similarly slight increase in patency rate was observed.
Implants with surface irregularities have been shown to promote aggressive macrophage activity. The effects of textured surfaces on interfacial cells, namely mononuclear phagocytes, include a change of infiltration of these cells, an increase in adhesion, vacuolization, filopodia formation, cytoplasmic-to-nuclear ratio, metabolism and enzyme activity. Foreign body giant cell formation augmented further the intensity of this response and indicated poor tolerance of the host tissue to the implant. Comparison of the tissue responses of abraded surfaces to smooth surfaces reveals a similar local increase in the population of macrophages around the abraded implants. In addition to being proliferative, these cells secreted high concentrations of enzymes such as leucine aminopeptidase, which suggested rapid tissue turnover.
Surfaces which were effective in the promotion of tenacious bio-adhesion required critical surface tensions between 30 and 40 dynes/cm. Fibroblast adhesion, spreading and growth were directly related to surface free energy of hydrophilic materials such as titanium, gold, nickel, hydroxylapatite, and glass.
In some cases, the contact angles of hydrophobic materials which are immersed in protein-containing serum appear to converge relatively close to that obtained for hydrophilic materials. A few well known biomaterials, carbon, poly-methylmethacrylate (PMMA), poly-tetra-fluoro-ethylene (PTFE) and PET, possess such ability. However, in a recent study of the response of rat tail fibroblasts, it was found that the change in surface energy of these substrates had no significant effect on cell morphology or growth rate. Instead, the study identified fiber diameter and surface topography of these materials as the two factors which influenced the morphologies of the fibroblasts.
A porous PET velour textile has been used as a replacement for blood vessels, and other textile biomaterials have been employed in various applications of soft tissue prostheses. There are a number of anatomical sites where PET velour fabric is now commonly used as a soft tissue replacement material. The local tissue response to PET velour fabric has been investigated at percutaneous, subcutaneous, and vascular implantation sites. In general, the tissue bed forms a connective tissue capsule around the implant. During the healing process, tissue extends into the implant's interstices and thus causes a direct attachment of the implant to the capsule, anchoring the implant to the host tissue. Normally, this anchoring is not observed with solid implants.
However, combined light and electron microscopic studies of the soft tissue in growth for PET velour fabric have demonstrated a high degree of cellularity, a scarcity of fibroblasts, and the presence of a delicate connective tissue matrix in the implant interstices, even after prolonged implantation periods in the subcutaneous tissue. This ingrown granulation tissue was found to be neither adherent to nor supportive of the individual PET velour fabric fibers and did not fulfill the initial design purpose of implant anchorage.